Vehicle suspension systems are well known in the art. Such suspension systems have as their goal the control of the relative motion between the sprung mass (the vehicle chassis) and the unsprung mass (the suspension arms, wheels, tires, etc.) of the vehicle. Passive suspension systems, such as shock absorbers, absorb and dissipate some of the energy and motion produced in an automotive vehicle by road surface irregularities when a vehicle is driven over such irregular road surface.
Passive suspension systems provide good vibration isolation at a relatively narrow range of vibration frequencies. A linear response of a passive suspension system can be altered by (i) adding an advantageous nonlinear attribute, such as direction dependant damping, and (ii) minimizing an objectionable effect, such as stiction, i.e., stick slip friction characteristics of the suspension system including the shock absorber. Passive systems, however, react only to applied forces from below the unsprung mass, i.e., from the road surface, and from above the unsprung mass, i.e., from inertia of the sprung mass or vehicle body. Ideally, the suspension system should appear "soft" in reacting to road surface induced inputs, ("road noise") and stiff when reacting to vehicle body accelerations and motions, ("inertia inputs"). Since a passive system cannot distinguish between the origin of the two types of vibration, an engineering compromise is made.
An active suspension system uses power from the vehicle engine to actively move the vehicle wheels over an irregular road surface usually with the objective of maintaining a constant force between the vehicle wheel and the road surface. Rather than a shock absorber, as is found in passive suspension system, an active suspension system uses a hydraulic servo-actuator, i.e., a hydraulic motor, to move, or control the forces in, the vehicle wheel. A plurality of sensors, for detecting impact, force, acceleration, velocity and displacement, are located at various vehicle locations. A controller, e.g., a microcomputer, monitors the sensor outputs and controls operation of the hydraulic actuator for the vehicle wheels and suspension located at each vehicle corner through an associated electrically controlled hydraulic servo valve. Through a control algorithm, the controller controls reaction to road noise and inertia inputs and controls relative motion of the sprung and unsprung masses.
In an active suspension system, the servo valve, actuator, and controller function as an energy control device. The servo valve connects the energy source, i.e., a pump, to the energy consumer, i.e., an actuator. The difference between power in and power out is converted to heat energy by the servo valve.
In a fully active suspension system, the actuator is operated so as to move the wheel up and down relative to the vehicle body as necessary to provide a desired "ride feel" and "handling characteristic" of the vehicle. The hydraulic pump provides energy in terms of fluid flow at system pressure. The servo valves remove energy at a rate to provide fluid flow and pressure so as to move an associated wheel at a velocity needed to achieve a desired ride feel and handling characteristics. Control of fluid flow with the servo valve controls actuator direction of movement and velocity. Control of fluid pressure, in turn, controls actuator force. The control signal output to the servo valve is referred to as a valve drive signal.